1. Field of the Invention
The present invention relates to an image processing apparatus and method, a recording medium, and a program. More particularly, the present invention relates to an image processing apparatus and method capable of correcting shading (sensitivity nonuniformity) of a captured image in a case where, for example, a subject is recorded by a video camera having a digital still camera function, a PDA (Personal Digital Assistant), a cellular phone, a PHS (Personal Handyphone System) or the like, to a recording medium therefor, and to a program therefor.
2. Description of the Related Art
In an imaging apparatus having a lens system and an imaging section, in general, it is known that shading (sensitivity nonuniformity) occurs in a captured image due to, for example, a reduction in the amount of peripheral light due to a lens system.
In recent years, as the cell size of imaging devices has been minimized, the construction in the height direction of wiring of imaging devices has been relatively minimized, and the pupil distance has been shortened as a result of a size reduction of lenses, causing shading to likely occur in a captured image.
The principles of this shading are described in, for example, Japanese Unexamined Patent Application Publication No. 2002-218298 as follows. In the imaging device, in order to improve light sensitivity for each pixel, a microlens is provided for each photosensitive pixel portion of the imaging device. When light beams passing through an image-capturing lens nearly vertically enter the photosensitive section of the imaging device, the incident light beams are collected on the photosensitive section of the imaging device almost without any problems. On the other hand, when the light beams passing through an image-capturing lens obliquely enter the imaging device, due to the optical relation between the image-capturing lens and the microlens, only some of the original incident light beams enter each photosensitive section of an area away from the optical axis of the image-capturing lens (i.e., peripheral portions of the imaging device), and shading occurs.
This phenomenon becomes more intense as the pixel position on the imaging device becomes further away from the optical axis of the image-capturing lens. Furthermore, in the case of an imaging device of a large size, in some of the image-capturing lenses, the angle of the incidence of the light beams to the peripheral portion of the imaging device becomes larger. In this case, shading in which the sensitivity decreases occurs in the peripheral portion of the imaging device, where the incidence angle becomes larger, due to the position of the microlens, manufacture variations of on-chip color filters, the device structure of the imaging device, and the like.
As a method of preventing the occurrence of such shading, for example, the lens system may be designed with a large number of lenses. Such a lens system designed with a large number of lenses is expensive, and application in so-called consumer products is often difficult.
On the other hand, for example, when extraction of signals is performed in accordance with the XY coordinates as in devices incorporating semiconductor imaging devices, the image can be corrected by a digital process on the extracted signal. Accordingly, in the field of scanners, etc., hitherto, various technologies for digitally correcting lens shading, such as distortion which occurs because photo-taking is performed by an inexpensive lens system, reduction of the amount of peripheral light, or color blur, have been proposed (see, for example, Japanese Unexamined Patent Application Publication Nos. 11-355511 and 2000-41183).
However, all of these conventional technologies are performed by being restricted to the field of scanners, etc. (for example, a considerable time can be taken for correction processing), and correction processing in real time is not required as in a digital still camera.
In comparison, various technologies relating to an imaging apparatus and a shading correction method for eliminating sensitivity nonuniformity on the plane of the imaging device, and an imaging apparatus and a shading correction method for correcting color shading which occurs in specific color channels of the imaging device (see, for example, Japanese Unexamined Patent Application Publication Nos. 2002-218298 and 2000-41179) have been proposed.
According to Japanese Unexamined Patent Application Publication No. 2002-218298, shading correction coefficients corresponding to pixels which are arranged two-dimensionally on the plane of the imaging device are stored in storage means, so that the correction coefficient is corrected so as to correspond to at least one of the zoom position of the image-capturing lens, the focus position, the image height, the aperture stop value, the exit pupil position, and the amount of strobe light emission.
According to Japanese Unexamined Patent Application Publication No. 2000-41179, shading correction data is calculated by a CPU (Central Processing Unit), the data is written into rewritable recording means (for example, RAM (Random Access Memory)), shading correction data is read in accordance with a captured image signal read from a CCD (Charge-Coupled Device), and shading correction is performed by multiplying the captured image signal by the digital signal after the photographic signal is converted from analog into digital form, thus making it possible to perform high-speed and accurate shading correction. Furthermore, as a result of being configured in such a manner that a program used for computations by the CPU can be changed externally, an image input device capable of performing shading correction of photo-taking corresponding to the scene of a subject (for example, corresponding to reverse optical correction and illumination) can be provided.
According to the technology of Japanese Unexamined Patent Application Publication No. 2000-41179, the number of types of the step of storing a shading correction coefficient corresponding to each of the pixels arranged two-dimensionally on the plane of the imaging device is one. For example, when the zoom position of the lens is moved from the wide end to the tele end, the correction coefficient needs to be corrected in such a manner as to correspond to each continuous zoom position.
Part A of FIG. 1 shows shading characteristics in the relationship between the amount of light (the vertical axis) and the position of the zoom wide end (the horizontal direction). Part B of FIG. 1 shows shading characteristics in the relationship between the amount of light (the vertical axis) and the position of the zoom tele end (the horizontal direction). As shown in part A of FIG. 1, shading characteristics L1 at the zoom wide end are gradual reduction of the amount of light, whereas, as shown in part B of FIG. 1, shading characteristics L2 at the tele wide end often have characteristics such that the amount of light decreases sharply at the peripheral portion of the image plane. As a result, problems arise in that it is difficult to correct the correction coefficient while moving the zoom position and to maintain the correction accuracy, and also in that the circuit scale and the correction program become larger.
The same applies to the focus position of the image-capturing lens, the image height, the aperture stop value, the exit pupil position, and the amount of strobe light emission.